It is well known to construct motor vehicle body doors, hoods, fenders, tailgates, trunk and deck lids by stamping an outer sheet metal panel and separately stamping an inner sheet metal reinforcing panel with an outer periphery generally matching that of the outer panel, and then joining the two panels together by hemming a flange of the periphery of the outer panel over an adjacent edge of the inner panel to secure the panels together. Desirably, the outer panel is slightly larger than the inner panel to provide a border flange portion along the periphery of the outer panel which preferably has an upstanding lip which can be folded over the peripheral edge of the inner panel to define the hem flange which connects the two panels.
Generally, to form the hem flange, a force of approximately 1,000 pounds per lineal inch is required. Thus, to form a hem flange about an elongate portion of a panel, a very large force is required. To withstand and accurately apply these large hemming forces, prior hemming machines have been large in size. So-called roller hemming machines utilize relative motion between a roller and the sheet metal panel to be hemmed to form the hem. The roller forms a hem about only a small portion of the sheet metal panel at any given moment, and thus, the force which the roller must apply to the panel to form the hem is greatly reduced. The reduced force enables the hem to be formed by more compact, more versatile and less costly hemming machines. Previous roller hemming machines generally have one or more rollers fixed on a frame or spring biased relative to the part on which the hem is to be formed. Thus, the roller hemmers provide a movement between the roller and the part to be formed in a first direction generally coincident with the edge of the part to be formed and possibly in a second direction generally in the direction of the spring biasing the roller. While satisfactory for generally straight, linear edges of a part, these previous roller hemming machines are not able to form a hem about more intricate parts having multiple curves such as sheet metal panels forming the doors, hood, trunk and deck lids and the like of modern vehicles.
One prior hemming apparatus, such as disclosed in U.S. Pat. No. 5,507,165, provides a tapered roller for pre-hemming the flange area of a panel which is carried by an arm pivotally connected to a slide and having a cam follower which engages a cam surface to pivot the arm so that the roller moves along a path substantially parallel to the flange. This hemming apparatus thus provides a compound movement of the roller in the general direction of the movement of the slide as well as in a direction dictated by the contour of the cam surface to form a pre-hem along the desired portion of the panel. Although useful for pre-hemming the flange of many parts, this hemming apparatus provides only a limited compound movement in a single plane which is insufficient to form a pre-hem of a flange about more intricate automobile body panels with peripheral edge portions having a compound curve.